DE102020005941A1 - Automatically changing termination for cables and wires - Google Patents

Abstract

In order to determine cable parameters in a real technical environment, the line speed vL and the characteristic impedance ZL of a line pair on the cable can be measured and other parameters can be determined from these measured values. The fact that a cable is reflection-free precisely when the end of the cable is terminated with the characteristic impedance can be used to determine the characteristic impedance. An adjustable resistor can be used to look for this resistance value. Because the reflections belonging to a line pair in an oscillogram are often difficult to distinguish from a disturbed signal background, both in terms of their time position and the respective reflection amplitude, and reflections from parasitic line pairs often are difficult to identify, a search adjustment of an adjustable resistor required for non-reflective properties is often difficult when only an oscilloscope or an oscillogram of the voltage at the cable input is available to check. If there is a unit to be connected at the end, which provides at least two different types of termination for a connected cable and which constantly switches automatically between these types of termination, the control required for a setting can also be carried out by displaying the signals on the oscilloscope . Such an exchange termination makes signal sections recognizable by area-enveloping diagram sections in the oscillogram, which allow a clear identification of reflections and a clear assignment to exactly one line pair. These area-enveloping diagram sections also allow a temporal position assignment of reflections with regard to the amplitude and the time position and only then also the amplitude classification during the setting of an adjustable resistance. The metrological determination of cable properties and cable parameters of a real cable, control during the design and selection of a cable when building and setting up bus systems, also taking into account and including parasitic line pairs is very easy.

Description

The invention represents a cable termination that automatically changes the type of termination, which is used, among other things, in connection with a pulse generator to determine the characteristic impedance of a cable or a pair of lines (hereinafter referred to as cable, without restricting the application to cables alone) and to determine the line speed V _L can support on a cable.

For a theoretical analysis and description, cables are i.a. in the form of a two-core cable, i.e. by two lengthways and i.W. parallel cable cores inserted. Thought to be replaced by infinitesimally small electrically discrete elements, properties are described by differential equations (DE; line equations). However, the solutions of this differential equation are not only seen as mathematical constructs, but are also interpreted as the properties of a real cable. Properties of a real cable are equated with the solutions and justified.

When an electrical signal is fed into a cable, the signal is at least partially reflected at the end of the cable after it has passed through the cable if the termination at the end of the cable does not correspond to the respective characteristic impedance of the cable. The reflected signal portion runs back in the cable to the feed point, where it may be reflected again (possibly more than once). The signal can also be at least partially reflected by disturbances or impedance changes in the course of the cable.

In practice, the theoretically described behavior of the reflection is confirmed very well in examples such as antenna cables, coaxial cables, but also in completely different types of cables and lines. The respective termination or the type of termination at the end of the cable, i.e. the impedance at the end of the cable or a change in impedance along the route of the cable, is decisive for the reflected portion of the signal.

Termination, in the given context, is a technical entity (a component resistor, a terminator, a circuit, a system, etc.) to be connected to a cable end (defined by the beginning of each cable) or to the end of a waveguide, pair of wires, etc. ) understood, but also the physical quantity itself with which a cable or pair of lines is terminated (i.e. an impedance, a resistance quantity that is to be specified in the unit [Ω], etc.). Ambiguous in this way, a cable termination according to the invention should also be a technical unit to be connected to a cable end, but should provide at least two different terminations for a respective cable (quickly switchable according to the invention); Each of these different terminations (termination with the characteristic impedance, short-circuited, open, ...) can cause the type of reflection (i.e. the course of reflection, the reflection event, the reflection amplitude, etc.) to be different and should therefore be understood as a "type of termination" for conceptual differentiation and so called.

A (cable or wire pair) termination according to the invention is therefore a technical unit to be connected to a cable end, by means of which two or more types of terminations (terminations) are provided for the respective cable. The cable termination according to the invention (as a technical unit) can switch between the terminations provided in this way automatically and quickly (several times every second), and can thereby provide very different types of termination at one cable end, possibly changing very quickly. For conceptual delimitation, a cable termination according to the invention, as a technical unit to be connected to a cable end, can also be referred to as a “changeover termination” and/or as an “automatic changeover termination” according to the invention.

In the examples considered here (cf. ) the signals fed into a cable should be short pulses (1), without thereby restricting the application of the cable termination according to the invention to such signals. Measurements with pulse signals allow the conditions on a cable to be recorded under real conditions and the visual representation of the measurement results makes it easier to interpret the physical processes.

However, the arrangements according to the invention can also be used using other, e.g. sinusoidal or other signals.

• - mit offenem Ausgang (in ist die sich auf diese Abschlussart beziehende Zugehörigkeit jeweils mit a bezeichnet)
• - bei Abschluss mit dem Wellenwiderstand Z (... jeweils bezeichnet mit b)
• - mit kurzgeschlossenem Ausgang (... jeweils bezeichnet mit c)

The use of pulse signals also allows the basic reflection behavior of a cable to be recognized and classified on the oscilloscope: shows this for three defined termination types, using a cable as an example

• - with open outlet (in the affiliation related to this type of degree is marked with a)
• - when terminated with the characteristic impedance Z (... each marked with b)
• - with short-circuited output (... each marked with c)

The indicated in the measuring circuits of indicated oscilloscopes (28), the pulse generators at the respective cable inputs (29) and the respective cable (23) are the same in all three cases ac; the measurement circuits differ only in the type of termination at the end of the cable (24).

If the cable end (24) (such as in or in is open, then the reflections (20) show an essentially the same course as the pulsed signal (22) fed in; Diagram Mapping (25). If the end of the cable (24) is terminated with the characteristic impedance Z, no reflections can be observed ( ; Chart Mapping (26). If the cable end (24) is short-circuited ( ), then the associated (27) oscillogram c shows a reflection (21) which has an inverted, but otherwise essentially the same course as the pulse signal (22) fed in; Diagram Mapping (27)

Theory and practice agree well as long as the considerations are limited to two-wire cables. What appears to be simple both in theory and in the practical use of a cable (e.g. simply using two of the conductors in a given cable as a pair of lines for transmitting data and/or signals), can quickly become complicated , if there are several conductor strands (cable cores).

A shielded two-core cable, i.e. a long cable strand with two internal conductors insulated from one another and additionally encased in a common shield, has not just one pair of conductors, but already three: The two internal conductors are generally the first pair of lines used for the transmission of data and signals; however, each of the two wire cores forms, together with the shielding, a further pair of wires which is to a certain extent classified as “parasitic” with respect to the first pair of wires.

A pair of lines should be understood here (and will continue to be referred to below) as only parasitic if this pair of lines should not actually play any role in the use of the first pair of lines that is the focus of consideration, but can inevitably have an influence (generally disruptive).

These "parasitic line pairs" generally have has a different characteristic impedance than the first line pair, but each line pair, i.e. each line pair combination considered individually and on its own, also shows the phenomenon of reflections at a cable end that is not terminated with the characteristic impedance.

With a coaxial cable this is easy; there are simply only two lines, i.e. only a single pair of lines. The prerequisite that theory and reality are comparable are largely given. However, the use of a pair of lines in a multi-core cable (also e.g. with a flat ribbon cable) can certainly become problematic, because the respective environment of a line of the line pair can play a not inconsiderable disruptive role.

It can often be seen in technical systems that an existing cable termination only refers to the (first) line pair intended for data transmission; the environment is ignored. However, it is not only this one degree that is to be interpreted correctly; In order to really achieve freedom from reflection, in this example of a two-wire shielded cable, the terminations for all three wire pairs must be correctly designed. This must be taken into account when planning, designing and building a bus system, for example, and even when selecting the cable and when designing such a cable. It is often not enough to consider the characteristic impedance of a line pair in a cable alone and to choose the right termination for this line pair with this characteristic impedance alone; the to a certain extent only parasitic pairs of lines must also be terminated with their characteristic impedance.

In this context, it is interesting that this extension of the consideration of a technically given cable can initially also focus on seemingly irrelevant cable details and cable usage details: For example, the twist pitch of line pairs to eliminate B-field couplings and/or the question of a GND reference, which repeatedly triggers fundamental discussions. Whether the shielding of a cable is better connected to GND at the beginning or end of the cable, or better at both ends, is a question that is no longer so irrelevant (but can be solved according to the invention).

When building a real system, however, i.a. often the possibility for a relevant metrological classification (another reason for not even considering details at all).

In order to also be able to classify (and correctly terminate) the additional parasitic line pairs in an existing cable construction, also in order to be able to evaluate an existing or selected cable termination, there is i.e. the need for tools with which the cable parameters of a given cable, in particular the characteristic impedance and the signal propagation speed, can be determined.

As explained, cable properties are represented by the solution of the above differential equation, in which the inductance and capacitance per unit length (L', C'), additional differential resistances (R', G') determine the essential properties of a cable. Among the properties that can be detected macroscopically from the outside are the characteristic impedance Z _L and the line speed v _L . The cause of the capacitance per unit length C' is considered to be the capacitance development between the line cores of the line pair, with the insulating material in between acting as a dielectric. L' and R' are parameters that can be assigned as a property to any electrical line. L' is not easily amenable to measurement for a given cable.

The line speed v _L (propagation speed) on a line pair can basically be determined directly with an oscilloscope by measuring the time offset of signals at two points along the line path. The achievable resolution is determined by the oscilloscope and the reading accuracy. Alternatively, a phase shift of periodic signals can be measured, which depends on the line speed v _L and the length of the cable (depending on the propagation time). This phase shift can easily be determined with very high accuracy down to the picosecond range.

If the characteristic impedance Z _L of a real cable can be measured, then from the values for Z _L and v _L measured on the real cable, following the theory, also e.g. L' = Z _L /v _L and C' = (v _L Z _L ) ^-1 can be determined for this real cable.

In this context, the object of the invention is to specify a method and an arrangement with which the characteristic impedance of a cable can be easily determined or classified in order to determine the terminations necessary for a reflection-free termination for cables and conductor pairs - including for parasitic conductor pairs and/or to be able to determine cable parameters.

• - sich diese gegenüber Signalen wie Wellenleiter mit der Fähigkeit zur Reflexion von Signalen verhalten, dass
• - das Reflexionsverhalten i.W. durch Impedanzwechsel im Leitungsverlauf bzw. durch den Leitungsabschluss bestimmt wird und dass
• - ein Wellenleiter reflexionsfrei ist, wenn der Abschluss mit dem Wellenwiderstand Z erfolgt.

The determination of Z _L =SQRT(L' / C') (SQRT... = square root of ...) and thus the solution of the task takes place by using the property of cables (wire pairs, etc.) that

• - these behave towards signals like waveguides with the ability to reflect signals, that
• - the reflection behavior iW is determined by impedance changes in the course of the line or by the line termination and that
• - a waveguide is reflection-free if it is terminated with the characteristic impedance Z.

The inventive use of the inverse conclusion is essential, namely that a cable whose cable end is terminated with an adjustable resistor is non-reflecting when the adjusted resistance value corresponds precisely to the characteristic impedance Z.

To do this, you have to find the state that is reflection-free by setting the adjustable resistance at the end of the cable, you have to measure the (ohmic) resistance that is set for this and you have also determined the characteristic impedance of the corresponding conductor pair.

However, the idea and approach according to the invention is also based on the experience that it is usually difficult to set the adjustable resistance for freedom from reflection if only an oscillogram or an oscilloscope that shows the reflection process is available for control purposes.

In the following, an oscilloscope is to be understood as the measuring instrument with an x and y deflection that is customary in an electronics laboratory, but also any method that can display a time-related voltage signal curve, e.g. recorded directly with a measuring tip, on a screen (as an oscillogram).

The one with an oscilloscope (10) (cf. The function curve that can be seen or shown at the beginning of the cable (2), which shows both the signal (pulse signal) placed on the cable by the generator (12) and the various reflections as a curve, should be referred to as the reflection curve or reflection representation. Since these reflection representations are not necessarily to be seen as a mathematical function, but as a more general curve (because the curve may represent more than just a functionality such as excitation signal and reflection), the term curve or curve should be used here to distinguish between them.

Such a curve (cf. can be clearly assigned to the one cable or pair of conductors (13) on which the measuring tip (19) is located; the assignment of signal elements (e.g. pronounced spikes) (5) (6) (18), the can be clearly seen in the course of the curve is much more difficult: these signal elements (5) (6) (18) could be reflections on this cable (13), but could also simply be artificial ringing or could be reflections originating from parasitic wire pairs , etc..

If two reflection courses are considered, eg for the termination type short circuit (i.e. R=0, smaller is not possible) and open (i.e. R=∞, larger is not possible), then the associated reflections are at the same points in time, but have different amplitudes. The curves belonging to different types of termination or the different oscillograms form, shown together (cf. right or ), at the point of a reflection, so to speak, an eye out ( , (63)). Since the term eye diagram has a different meaning in the technical environment, it will not be used here.

In this small interval range covered in this way, all other reflection courses on this cable will remain at final values 0 < R < oo, also R=Z.

It is important that these enveloped interval ranges of different types of closure i.a. only result in at least two reflection profiles that are clearly assigned to the cable on which the measuring tip is located and on which the change in the type of termination takes place.

When terminating with the characteristic impedance at the end of a cable, there are no reflections ( . In this case, only the fed-in pulse (22) in the oscillogram (b) is for the measurement arrangement yet to see.

With a switch it is possible to set the states open and shorted switchably, like that displays. But even then you will only ever see a curve from one of the two types of termination individually on the oscilloscope. To see the reflection events of the two types of terminations simultaneously, or a superimposed representation (32) of shorted and open output, like the one with shown on the right is generally not possible directly. An overlay mode required for this is generally not provided, and a storage oscilloscope is not always available; therefore, in general, only a single trace will always be visible; either the oscillogram of a signal with reflections from the cable when the cable end is short-circuited, or the oscillogram of a signal with reflections when the cable end is open.

However, a pictorial overlay can also be provided, at least for a human observer, with an oscilloscope if it is possible to switch between the different terminations quickly enough: if every second, for example, at least three or more times between several termination forms (e.g. the open, short-circuited and with cable end provided with an adjustable resistance) is changed, then due to inertia of the eye, the human observer also sees this as a superimposed image of the various forms of reflection at these terminations.

In order to see the reflections as a stable picture on the oscilloscope, the pulse frequency must be greater than this switching frequency. This defines relatively low requirements for the pulse generator (12), which provides, for example, 10ns pulses (1) with, for example, a frequency of approx and adjustable resistance changes the termination for a given cable.

Depending on the respective application, an automatically executed change can take place at any time intervals, e.g. at regular intervals of a period (e.g. a second or a fraction thereof) between the several types of trades at regular intervals, but can also be carried out at regular intervals of a period Change deal types at uneven time intervals. This means that a change in the type of termination can take place at any rapid time interval, provided that the speed of the change makes it possible to record or perceive the reflection process from several types of termination at the same time.

Due to the fact that at least two different types of termination for the respective cable are provided quasi-simultaneously by the automatic switch termination, new picture elements are created that may not otherwise be perceptible in the normal oscillogram, such as an interval-enveloping (30) (31) curve (eye diagram) , which are only accessible in the first place. This enables the temporal position allocation of reflections and also enables the amplitude classification that is necessary during an adjustment.

Due to the fact that reflections with an open cable end ( ) a positive image (20) of the applied pulse excitation signal (29) (22), at the short-circuited cable end ( ) represent an iW equally high inverted image (21) of the applied impulse excitation signal (29) (22) and these types of reflection are at the same time interval from the impulse excitation signal, forms with a suitably fast termination change by the Inertia of the observer's eye (or through technical image processing) in the superimposed pictorial representation (32) of the two signal curves (30) (31) only at a temporal reflection point that can be clearly assigned to the cable termination according to the invention (the area (63)) enveloping curve .

• - die nicht durch einen wechselnden Kabelabschluss beeinflusst sind oder
• - Reflexionen, die von anderen (z.B. parasitären) Leitungspaaren stammen und dadurch ebenfalls nicht vom Wechselabschluss beeinflusst werden,

bleiben auch in einer solchen überlagerten Darstellung (32) i.W. gleich (vgl. in z.B. (61) und (62), s.u.) und bilden daher auch keinen den Bereich (63) derartig umhüllenden Kurvenzug aus.Other signal elements or points in the curve (function curve),

• - which are not affected by a changing cable termination or
• - Reflections that come from other (e.g. parasitic) line pairs and are therefore also not influenced by the alternating termination,

remain essentially the same in such a superimposed representation (32) (cf. in eg (61) and (62), see below) and therefore do not form a curve that envelops the area (63) in this way.

It is important that only by superimposing at least two different reflection profiles ( and ) a border of an area (63) that can be clearly and unambiguously delimited from the rest of the curve (cf. ) arises, which can be clearly assigned to a reflection that clearly belongs to the pair of lines to which the respective exchangeable termination according to the invention is connected. The curve enclosing this area was and will be further referred to below as “enveloping” or as an enveloping curve, which defines both a time interval and an amplitude interval.

At this point it should be noted that a calculated difference formation using pairs of reflection courses can also highlight these areas with exactly one reflection, such as e.g . However, this requires a digitized recording of at least two curves (with analog-digital conversion and storage), a subsequent calculation of the difference (using a computer) and then the conversion and display of the result on a monitor. In the state of the art, this naturally represents a conceivable way of processing the data that arises and can also be used to display and highlight reflections and reflection points; however, this does not depart from the scope of the invention.

The use of (analogue or digital) storage oscilloscopes does not leave the scope of the invention either, since an exchange termination according to the invention is also necessary here; however, the exchange speed can then possibly be greatly reduced.

The invention is based on until are explained: These are the ones already used in the state of the art and generally represents the technical conditions or the reflection events on a cable. shows the reflection curves for the termination types "open", "short circuit" and "termination with the characteristic impedance" and what a superimposed representation can emphasize. until deepen the importance of an adjustable resistance termination. Based on it can be shown that an embodiment of the cable termination according to the invention can also be used when the generator pulse and the reflection overlap in time. shows an embodiment with the reflection profiles during an adjustment of the variable resistor.

The impulses shown in the examples, e.g. in , (3), are about 10ns wide (cf. in on the right the 100ns scaling (15)) and follow each other at intervals of 10µs to 20µs (7) or faster or slower. The generated impulses (1) follow one another with a defined amplitude (8) so quickly that the impulses (1) can be seen on an oscilloscope screen as a stable, bright image even with a very high time resolution. The pulses (1) (3) of the generator are well formed in themselves, but are deformed by the pre-processing and the limited bandwidth of the oscilloscope and the probes, and in this example they also oscillate. (See the measurement circuitry in below with the enlargement (11) of the image seen on the oscilloscope). However, this is not seen as a disadvantage for the presentation of the invention; in the case of applications of a cable termination according to the invention, limited measurement options can/must be manageable. De-embedding is generally not necessary either, but it should be noted that the measuring arrangement also has an influence on the display on the oscilloscope (10). For example, in the recording of the , in which only the generator (12) and a measuring tip (19) with a connecting cable (17) are connected to the oscilloscope (10), reflections (9) can already be seen, although no cable (13) is connected at all; these come from the cable (17) of the measuring tip.

If this pulse signal is observed at the feed point (by definition, the beginning of the cable (2)), e.g. with an oscilloscope (10), then the fed pulse signal (3) occurs after a time t _L (4) (here 45ns), which corresponds to the transit time of the signal over twice the cable length (there and back, e.g. 9m) corresponds to seeing reflections ((5), possibly also (6), , (1), (3)) if the termination at the end of the cable does not exactly match the characteristic impedance. Whether the in this chart with (18) are also reflections, cannot be clearly decided from a single display on the oscilloscope.

To determine the characteristic impedance Z _L of a cable (13) ( As stated, a pulse generator (12) is used, which generates very short pulses (eg ion pulses). The pulses (1) (3) generated by the generator (12) are applied to the input of the cable (2), run through the cable (13) to the cable end (16), which is specifically terminated with a termination (14) (here as an individual component can be seen) is terminated, are reflected (or not reflected) there depending on the respective termination type (14) and run back over the cable section (13) to the beginning of the cable (2).

A termination as a technical unit to be connected to a cable end can provide or connect specific resistance values (R=0, R=∞, R=Z, etc.) and switch the resistances as a termination on a cable end.

If the voltage at the beginning of the cable (2) is observed with an oscilloscope (10) (for which a measuring cable (17) with, for example, a measuring tip (19) is required), both the generator signal (3) and the signal on the signal are offset in time ( 4) incoming reflections (5), possibly (6) and possibly also (18) can be seen.

• zeigt das Verhalten der Messanordnung bei offenem Ausgang: Im Oszillogramm (a, oben, zugeordnet durch (25)) ist der auf den Kabelanfang (29) gelegte Generatorimpuls (22) zu sehen und nach einiger Zeit die Reflexion (20). Diese Reflexion (20) zeigt im Prinzip den gleichen Verlauf wie das Generatorsignal (mit einer positiven Amplitude) und liegt hier in einem zeitlichen Abstand von t_r= 45ns. Die Leitungsgeschwindigkeit v_L auf diesem Kabel ist in diesem Beispiel 2*4,7m/45ns = 209.000km/s, beträgt auf dem Beispielkabel also ca. 2/3 der Vakuum- Lichtgeschwindigkeit.

shows the example of a coaxial cable already used above with three possible termination types to be seen as borderline cases and thus the basic behavior of this arrangement:

• shows the behavior of the measuring arrangement with an open output: In the oscillogram (a, above, assigned by (25)) the generator pulse (22) applied to the beginning of the cable (29) can be seen and after some time the reflection (20). In principle, this reflection (20) follows the same course as the generator signal (with a positive amplitude) and is here at a time interval of t _r =45 ns. The line speed v _L on this cable is in this example 2*4.7m/45ns = 209,000km/s, so it is about 2/3 of the vacuum speed of light on the example cable.

shows the behavior (of the same arrangement) with a short-circuited output: In the oscillogram (c, below, assigned by (27)) the generator pulse (22) can be seen again, which is fed in at the beginning of the cable (29). At the same point (after the same time t _r =45 ns), a reflection (21) of the signal from the end of the line (24) can be seen again. However, the reflection (21) now shows an inverted course, ie a negative amplitude.

shows the behavior of the same arrangement with an output terminated with the characteristic impedance. In the oscillogram (b, middle, assigned by (26)), the generator impulse (22) can be seen again, which is placed on the cable (29), but at the point at which there was a reflection (20) in each of the previous examples. (21) was seen, there is now no more reflection.

With open ( and shorted ( ) Output (24), the fed-in signal (22) and the reflections (20) (21) in the oscillogram are therefore at the same time points at a distance of approx. t _L =45ns. If you put the oscillograms of the and in a common display (32) on top of each other (30) (31) (here the one generated by image superimposition on the computer right) then the same temporal position of the reflections can be seen. You can see that these reflections are approximately the same in terms of amount and you can see the formation of an eye diagram (63) or a curve that envelops a defined area or the “envelope”.

raises the part of the oscillogram display right, in which the relevant reflection curves are located, and shows a switch (33) in the measurement circuit indicated below, with which the type of cable termination can be changed (switched) between "short circuit" and "open cable". The oscillogram shows that the boundary reflections (open and short) are about the same. This means that with regard to the zero line, the (positive) reflection with an open cable output has approximately the same amplitude level (34) as the (negative) reflection (35) in the other direction with a short-circuited output. Both oscillograms or curves superimposed define both a time interval in which all other reflection profiles of terminations with different ohmic termination values will lie and an amplitude range in which all other reflection profiles for terminations with different ohmic termination values will be.

In addition, one can now (compared to the representation of the with only one oscillogram) and classify other signal parts, especially reflections: The in with (6) (in reflection denoted by (61)) is also in clearly visible, but certainly does not come from the line pair to which the switch (33) is connected, since no change can be seen here when switching from short circuit to open and vice versa. On the other hand, the later small reflection, which would probably otherwise not be considered (in the second part of (18), in denoted by (62)), now recognize bar very clearly from what is happening on this line pair. Because the enveloping curve (the eye (62)) can be seen by switching over, the assignment can (but only now) be made unambiguously. The formation of envelopes is therefore a characteristic property of what is happening on a line pair that is terminated with an exchange termination according to the invention.

shows an oscillogram overlay of several reflection curves obtained with the same technique (image overlay), in which an adjustable resistor (a potentiometer) (38) is selected as the termination and different values are set: When the potentiometer is set to R=0 (short circuit) the inverted complete reflection can be seen as an extreme value, ie the lower curve (37). When the resistor (38) is set to the maximum value of the potentiometer (here R=2kΩ), a positive reflection (36) can be seen, which is almost (but not quite) as high as with an open output with R=∞. With a potentiometer setting that corresponds to the characteristic impedance (here 75Ω), no reflection can be seen (39). With other setting values on the potentiometer, there are other intermediate values, but always between the two outer curves (36) (37), which, despite the unsteadiness of the curve (or function) progression that can be seen overall, are easily recognizable as reflections and can be well classified by the human observer both with regard to their temporal position and with regard to the set amplitude.

There are four reflection curves that are relevant as limit values and are used as a special type of termination: They belong to the (ohmic) terminations R=0, R=∞, R=Z and R=R _P ( ). R _p is the maximum value that can be set with the potentiometer at the end of the cable and generally corresponds to the nominal value of the adjustable resistor or potentiometer, eg 2kΩ. The amplitudes ( or. ) of the reflection curves to R=0 and R=∞ are approximately symmetrical to the zero line and thus form an interval-enveloping curve section (an envelope or an eye) in which the reflection curves will remain at every other (ohmic) termination. If a resistor (eg 2kΩ) is connected to the cable output, the amplitude of the fed-in signal drops somewhat (41) (42), and as a result also the amplitude of the positive reflection (40) (43). The signal curve reduced by R _P =2kΩ and the reflection height already reduced as a result can be ignored. The only important thing is that when terminating with a 2kΩ potentiometer, the short-circuit setting can be set with R=0, but not with a completely open state (R=∞) and that the overall reflection levels are somewhat reduced, but the characteristic impedance can still be adjusted with the potentiometer is.

In summary, it can be stated that oscillograms in the prior art are always present individually on an oscilloscope without downstream image processing and without an overlay mode, and it is therefore difficult to recognize and achieve a desired non-reflection by adjusting the potentiometer.

An important point of the application of an exchange termination according to the invention can be seen that with a given cable not only the termination of the i.a. first line pair provided for signal or data transmission can be evaluated, but now also the terminations of parasitic line pairs. For this purpose, for example, a termination according to the invention can also be assigned to a parasitic line pair and the characteristic impedance, as described above, can also be determined from this parasitic line pair and the parasitic line pair can in turn be terminated reflection-free with the characteristic impedance determined in this way. Unanswered questions can be clarified in advance: For example, is it better to shield two lines individually or is it sufficient to have a common shield; how does the pitch of a twist affect it; how and where must the GND reference be made, etc.

Only when all line pairs, including the parasitic line pairs, are terminated without reflection can interference from reflections on lines and cables really be ruled out.

If the signal of a pulse generator at the beginning of the cable and the reflections from the end of the cable can be seen with an oscilloscope and a resistor can be adjusted at the end of the cable in such a way that there is no reflection, then with this adjusted resistor the characteristic impedance Z _L =SQRT(L'/C') of the cable, which is then assigned to this cable pair as a cable termination.

In order to enable or at least make it easier to set the terminating resistor, the termination of the cable is changed - repetitively at relatively short intervals - so that the viewer on the oscilloscope can perceive the associated reflection curves practically simultaneously (e.g. for short-circuit, open and Z as a type of degree).

shows the basic circuit of a termination (60) according to the invention at the bottom right. All switches with low contact resistance are suitable for the switches provided, too eg semiconductor or MOSFET switches. The assembly is connected to a cable with the left connectors (60'). Depending on the switch position, the termination for the cable connected in this way is open (eg in the state shown), short-circuited (if both switches are in the complementary state) or is defined by the adjustable resistance.

A preferred application of such a cable termination according to the invention is with time domain reflectometry. With such measurements, it can make sense to design a cable termination that can be switched automatically according to the invention in such a way that additional or specific values of terminating resistors, e.g. for limit value specifications, can be used.

A cable termination according to the invention can generally also have termination resistance values for representing limit values or also value limits. Terminations according to the invention can thus have at least three types of termination between which the cable termination can switch automatically.

A setting range recognizable by enveloping curves can be made symmetrical in a targeted manner, for example by only adapting a “near short circuit” to the given R _p (e.g. 2kΩ) of the adjustable resistor with which a pulse amplitude at R=R _p is set as the maximum reflection value (e.g. by terminating with R=10Ω or P=5Ω). This can happen in such a way that the positive outer enveloping curve (36), which is smaller when terminated with 2kΩ than with R=∞, and the negative outer enveloping curve (37), which is smaller in terms of absolute value due to a termination with 5Ω, for example at R=0, have equal amplitudes (34) (35).

An adjustment range resulting in this way from two curves lying symmetrically to the anechoic state can, for example, no longer provide for the complete short circuit (37) and the open state (similar to (36)): are specified and thus any permissible values for the characteristic impedance value are displayed, then the range visible to the user on the oscilloscope can be expanded by suitably setting the y-amplifier of the oscilloscope; an adjustment of the potentiometer can be done more precisely and even easier; the position of cable properties or parameters with regard to defined limits can thus be classified visually for test purposes.

It should be noted that the reflections that are very clearly distinguishable in terms of time in the examples with regard to the test pulse signal do not necessarily have to be far apart from one another. The diagrams of show an example where the cable is so short that reflections already arrive at the cable input before the generator signal has even ended: The diagram at the top left (50) shows a voltage curve that can be seen on the oscilloscope with an open cable end, the middle diagram Above (51) shows a voltage curve with a short-circuited cable end, the upper right diagram (52) with a termination with the characteristic impedance Z. The theoretically expected curve is indicated above the respective oscillogram (57) (58) (59). If the diagrams (50) and (51) are shown superimposed in a common diagram (53) (55), then one can clearly see the envelope curve area from the curves (50) (51), with which a reflection-free curve progression (52 ) can be easily classified and assigned (56) or which is still easily recognizable for setting a resistor to the characteristic impedance value Z (54).

• Da die beschriebenen Beobachtungen am Oszilloskop, d.h. i.a. am Kabeleingang stattfinden, der Nutzer sich bei der Einstellung des einstellbaren Widerstandes aber am Kabelabschluss befinden muss, kann das bei Messungen an bereits gelegten längeren Kabelstrecken mühsam werden, wenn sich Kabelanfang und Kabelende in einer größeren Distanzbefinden. Hier kann ein Datenaustausch zwischen der Position des Kabelanfangs (der Position des Oszilloskops mit Impulsgenerators) und der jeweiligen Position des erfindungsgemäßen Kabelabschlusses sehr nützlich sein. Denkbar sind z.B. Steuerungssignale, um von der Position des Beobachters aus ein digitales Potentiometer im erfindungsgemäßen Kabelabschluss einstellen zu können. Ein digitales Potentiometer könnte z.B. statt eines analogen Potentiometers (in z.B. (38) oder z.B. (97)) im erfindungsgemäßen Abschluss vorgesehen werden.

A preferred application of such a cable termination according to the invention is time-domain reflectometry measurements. For this it can be useful to set up a data exchange between the position of the pulse generator used and the position of the cable termination according to the invention, which can be wired or wireless:

• Since the observations described take place on the oscilloscope, i.e. at the cable entry, but the user has to be at the cable termination when setting the adjustable resistance, this can become tedious when measuring on longer cable sections that have already been laid if the cable beginning and cable end are at a greater distance. Here a data exchange between the position of the beginning of the cable (the position of the oscilloscope with pulse generator) and the respective position of the cable termination according to the invention can be very useful. Control signals are conceivable, for example, in order to be able to set a digital potentiometer in the cable termination according to the invention from the observer's position. For example, a digital potentiometer could be used instead of an analog potentiometer (in e.g. (38) or eg (97)) can be provided in the conclusion according to the invention.

It is obvious that such a data exchange between the position of the pulse generator and the position of the cable termination during an adjustment at the cable termination can also take place by transmission of the oscilloscope data or images, because and/or if oscilloscopes can be connected to the intranet or Internet . However, it can then also be useful (cf. at the cable beginning (2) of a cable (13) on which the pulse generator (12) and normally also the oscilloscope (10) is located, the data that can otherwise only be viewed there with the oscilloscope in an additional unit (19) of the pulse generator (12) prepare and to the position of the user, for example, to a PC at the cable end (13) or a unit that includes the cable termination (14), to transmit and display there.

• - neben dem eigentlichen Impulsgenerator (12)
• - eine Digitalisierungseinheit (19) und
• - Mittel zur Übertragung von Daten aufweist,

um Daten an die Position eines erfindungsgemäßen Kabelabschlusses übertragen zu können.The pulse generator can thus itself represent a more extensive technical unit that

• - next to the actual pulse generator (12)
• - a digitizing unit (19) and
• - has means for the transmission of data,

in order to be able to transmit data to the position of a cable termination according to the invention.

The image overlay can then be composed of individual measurements at the end of the cable, e.g. also on a PC, and displayed there, also automatically.

It is essential that the curves required for a setting of the analog or digitally adjustable resistance for checking purposes are accessible to the user in real time during the setting. For this, for example, the limit curves (20) (21) in to be seen together; only then does a curve (or an eye) (40) (62) in , where all other reflection images to another cable termination reside; including the reflection image of the anechoic curve during a resistor adjustment.

• - Impulsgenerator (12) mit einem Ausgang zum Anschluss (2) an ein Kabel (13) und mit einem Ausgang (19) zum Anschluss eines Oszilloskops (10) mittels Messleitung (17), in Kombination mit einem erfindungsgemäßen Kabelabschluss (14), der einen einstellbaren Widerstand aufweist;
• - Impulsgenerator (12) mit einem Ausgang zum Anschluss (2) an ein Kabel (13) und mit einem Ausgang (19) zum Anschluss eines Oszilloskops (10) mittels Messleitung (17), mit Mitteln zur Datenübertragung an den (evtl. entfernt liegenden) Ort eines erfindungsgemäßen Kabelabschlusses, um dort
  • ◯ einen einstellbaren Widerstand steuern und einstellen zu können oder um dort
  • ◯ die Oszillogramme zur Verfügung zu stellen,
  in Kombination mit einem erfindungsgemäßen Kabelabschluss (14), der Mittel zum Empfang von Steuersignalen vom Impulsgenerator am Kabelanfang aufweist, womit
  • ◯ ein einstellbarer Widerstand eingestellt oder gesteuert werden kann, oder
  • ◯ übertragene Oszillogramme dargestellt werden können.

Among other things, the following arrangement combinations are useful (cf. :

• - Pulse generator (12) with an output for connection (2) to a cable (13) and with an output (19) for connecting an oscilloscope (10) by means of a measuring line (17), in combination with a cable termination (14) according to the invention has an adjustable resistance;
• - Pulse generator (12) with an output for connection (2) to a cable (13) and with an output (19) for connecting an oscilloscope (10) by means of a measuring line (17), with means for data transmission to the (possibly remote) ) Location of a cable termination according to the invention to there
  • ◯ to be able to control and adjust an adjustable resistance or to there
  • ◯ to provide the oscillograms,
  in combination with a cable termination (14) according to the invention, which has means for receiving control signals from the pulse generator at the beginning of the cable, with which
  • ◯ an adjustable resistance can be set or controlled, or
  • ◯ transmitted oscillograms can be displayed.

It should be noted that a cable termination according to the invention is also useful for determining the signal line speed v _L (in addition to determining the characteristic impedance Z _L ) of a cable due to the design of the envelope. By displaying the envelope in the oscillogram, it is possible to identify exactly where a reflection is actually present in terms of time. In the case of oscillating and/or disturbed signals or lines, reflections can be identified more easily because an envelope curve formation can only come from the pair of lines on which the termination according to the invention is located.

Because a pulse generator generates very short pulses as test signals, the reflections associated with a termination can be clearly identified and distinguished from other signal components by means of an alternating termination according to the invention. From this it follows that when determining the signal line speed v _L on a cable, multiple reflections can also be suitable. Based on For example, the transit time between the pulse excitation signal (42) and the first reflection, identifiable by the first envelope (40), was determined. The superimposed representation clearly identifies a second envelope (62), which can be assigned to the same pair of lines due to the cable termination according to the invention at the end of the cable and is the result of a cable input that is not terminated with the characteristic impedance (a reflection of the first reflection). Half the time interval between the two envelopes (40) and (62) corresponds to the one-time passage of the pulse generator signal through the cable.

Such an identification option by means of enveloping curve sections is particularly useful in the case of multiple possible reflections, e.g. when several cables are connected to a signal source or in the case of several serially connected cable pieces, etc. Here, however, this is particularly relevant in the context of classifying reflections for the respective line pair and/or to the parasitic wire pairs.

If both on a defined pair of wires of a cable, as well as on the resulting If an alternating termination is placed at the end of each parasitic line pair, then reflections can be quickly and specifically assigned to the different line pairs. In particular, it can be useful if there are communication and/or control options through data transmission between the beginning and end of the cable.

A cable termination of possibly several cable terminations can then be activated and/or deactivated in this way, for example.

The fact that it is possible in this way to automatically determine the required terminations for multiple pairs of lines and also for the resulting parasitic pairs of lines should only be mentioned in passing. With a suitable design, a suitably designed device can also learn all the necessary termination values for multi-core cables itself. For example, for the three line pairs of a two-wire shielded cable, which has two parasitic line pairs in addition to an actual data transmission line pair, the required terminations can be determined manually and/or automatically if communication and/or control options are possible through data transmission between the beginning and end of the cable consist.

It should also be noted that a cable termination according to the invention can also be used sensibly for determining faults on a cable in general time-domain reflection measurements due to the design of the envelope: Only the cable provided with a termination according to the invention shows a reflection event that at least two termination types can be assigned as an enveloping curve. Again, this can only be one that comes from the end of the cable, because that is where the cable change termination is located and changes automatically and periodically between two states. Thus, in a general time-domain reflectance measurement performed to detect imperfections on a cable, the reflections coming from the end can be uniquely identified by the appearance of the envelope curve, and any intervening impurity reflections can be isolated and uniquely assigned. In addition, the relative geometric position of the point of disturbance along the entire length of the cable can be classified exactly by the relative position in time of a point of disturbance reflection between the start pulse and the end point reflection. If the line speed is constant along the entire cable run, this can be done even without knowing the actual line speed.

shows a preferred embodiment of a cable termination according to the invention (top left) as a unit (connector (93)) to be connected to a cable end, which contains an adjustable resistor (97), two fixed resistors (95) (96) and four switches. The switches can be switched by a suitable control (not shown). The reflection images or signals that occur are shown.

To determine the characteristic impedance, a setting is sought with the adjustable resistor (97) in which the cable connected to (93) is terminated without reflection. In order to be able to explicitly determine the value of the wave resistance after the resistance has been set, it must be measured with an ohmmeter at the contact (94) provided for this purpose. The resistance measured at this contact (94) corresponds to the characteristic impedance.

The contact point (93) for the cable to be connected is preferably constructed in the same way as the contact point for checking the set resistance ((97) in series with (96)). This has the advantage that once the setting has been made, the termination according to the invention simply has to be turned around and connected to the end of the cable in order to terminate the cable with the characteristic impedance. This can be done without actually measuring the characteristic impedance with an ohmmeter.

The use of a cable termination according to the invention (top left) as a unit to be connected to a cable end takes place on the connected cable (connection (93) as follows: The switch positions shown represent the type of termination “open termination” for the cable. The switch on the far left changes its position, then the type of termination is "short-circuit“. If these switch positions are changed quickly, you can switch between short-circuit and open so quickly that the two reflection curves belonging to this cable are displayed in superimposed form on the oscilloscope. The enveloping curves that form are can only be clearly assigned to the pair of lines to which the cable termination (93) according to the invention is connected.At least one enveloping curve is formed, which defines an amplitude interval (98) and a time interval (99).

When the switch on the left is in the "open" position, the connected cable can be assigned with the three other switches (from top to bottom) as termination type: 1. the adjustable resistor (97) alone, 2. the adjustable resistor (97 ) plus a resistor (96) in series or 3. additionally the adjustable resistor (97) plus two resistors in series stands (96) (95). show the benefits and effects of these types of terminations when it is possible to quickly switch between them: For guidance, the reflections of the "open" state with the amplitude (100) and the reflections with the shorted termination with the amplitude (101) in are shown shown and labeled the same (because they are the same). The curves on the inside of these two outer reflection curves result from the superimposition of reflection images that arise during a running adjustment on the potentiometer. The reflection curves (100) (101) always remain the same.

When changing the switch quickly (changing the type of termination for the connected cable), the reflection images for the terminations R _Poti , R _Poti +ΔR and R _Poti +2ΔR are also formed.

shows a setting on the potentiometer where, with the currently set resistance value, there are resistance values that are greater than the characteristic impedance of the connected cable. The reflections (102) (103) (104) resulting from terminations R _Poti , R _Poti +ΔR and R _Poti +2ΔR are clearly present and all have a positive amplitude.

shows a setting on the potentiometer where, with the currently set resistance value, there are resistance values that are smaller than the characteristic impedance of the connected cable. The reflections (105) (106) (107) resulting from the terminations R _Poti , R _Poti +ΔR and R _Poti +2ΔR are also clearly present and all show a negative amplitude.

shows a setting on the potentiometer in which, with the currently set resistance value, exactly one curve (109) of the reflection images (108) (109) (110) shows a reflection-free state and the associated resistance value therefore corresponds to the characteristic impedance of the connected cable. R _Poti < (R _Poti +ΔR) < (R _Poti +2ΔR) applies. Since one of these reflection curves (108) is positive and greater than the zero line, this reflection curve can only have been created by ending with the resistance value that is the largest value of the three resistance values, i.e. R _Poti +2ΔR. Since one of the reflection curves (110) is negative and smaller than the zero line, the associated resistance value is the smallest of all three, i.e. R _Poti . The curve (109), which shows itself as reflection-free, is only present at the end with the characteristic impedance and this corresponds to the resistance value R _Poti +ΔR.

It is important here that the resistance combination (95) (96) (97) in connection with the fast switching according to the invention allows the characteristic impedance to be found by adjusting the potentiometer in that a symmetrical position of three curves can be set to a zero position. Even if there is very strong interference on the cable, this is relatively easy to do, especially since the curves (108) (109) (110) can be spread out by the oscilloscope y-gain.

Claims (10)

Cable termination (14) as a unit to be connected to a cable end (16) or to a line pair end, characterized in that the cable termination (14) for a connected cable (13) or line pair (hereinafter only cable) has at least two different terminations or Closing types (24) (33) (38) that can be switched between automatically. cable termination after claim 1 , characterized in that the cable termination (14) changes so quickly between the at least two terminations or termination types (24) (33) (38) that the oscilloscope (10) (28) changes the oscillograms of the reflection events (32') from different degrees or types of degrees overlaid (32). cable termination after claim 1 or 2 , characterized in that the cable terminator (14) for a connected cable (13) provides an open cable terminator (24a), a short-circuited cable terminator (24c) and at least one cable terminator which has an adjustable resistor (38) as a terminator, i.e. at least three Termination types provides between which the cable termination (14) can switch automatically or controlled. Cable termination after at least one of the Claims 1 until 3 , characterized in that the interval-enveloping curves (62) that form in superimposed images (32) of at least two different termination types (32') are used to identify reflection points for a given cable or line pair. Cable termination after at least one of the Claims 1 until 4 , characterized in that in order to form defined reflection heights (108) (109) (110), specially designed resistance values (95) (96) (97) are used for at least two types of termination, in order to create a visible limit value range (108) (110) on the Display the oscilloscope as an interval-enveloping curve during an adjustment. Cable termination after at least one of the Claims 1 until 5 , characterized , that the cable termination (14) is used in conjunction with a pulse generator (12) as an aid for determining the characteristic impedance Z _L and/or for determining the line speed v _L and/or for time-domain reflection measurements on a cable (13). Cable termination after at least one of the Claims 1 until 6 , characterized in that the cable termination (14) is used to determine the characteristic impedance Z _L and/or to determine the line speed v _L of parasitic line pairs. Cable termination after at least one of the Claims 1 until 7 , characterized in that the cable termination (14) forms two enveloping curves (108) (109) for delimiting and assigning reflections over time, and a terminating resistor (97) can be set in such a way that an oscillogram (109) freedom from reflection (109) displays. Cable termination after at least one of the Claims 1 until 8th , characterized in that cable terminations at the end of a multi-core cable are each associated with both a line pair and the line pairs forming parasitically thereto in order to be able to use the enveloping curves to allocate the forming reflections to different line pairs. Cable termination after at least one of the Claims 1 until 9 , characterized in that there is a wired or wireless data transmission between the beginning and the end of the cable to enable communication and/or control possibilities between the position of the pulse generator and the position of the cable termination.

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